Getting Data from Here to There: How Computers Share Data

Matt Hayden and Joe Habraken provide a brief run-down of the four most common types of network topologies: Ethernet, Token Ring, FDDI, and ATM. A Q&A section at the end provides answers to common questions about these topologies.

This chapter is from the book

This chapter is from the book

In this hour, we will first discuss four common logical topologies, starting
with the most common and ending with the most esoteric:

Ethernet

Token Ring

FDDI

ATM

In the preceding hour, you read a brief definition of packet-switching and
an explanation of why packet switching is so important to data networking. In
this hour, you learn more about how networks pass data between computers. This
process will be discussed from two separate vantage points: logical topologies,
such as ethernet, token ring, and ATM; and network protocols, which we have
not yet discussed.

Why is packet switching so important? Recall that it enables multiple computers
to send multiple messages down a single piece of wire, a technical choice that
is both efficient and an elegant solution. Packet switching is intrinsic to
computer networkingwithout packet switching, no network.

In the first hour, you learned about the physical layouts of networks, such
as star, bus, and ring technologies, which create the highways over which data
travels. In the next hour, you learn about these topologies in more depth. But
before we get to them, you have to know the rules of the road that determine
how data travels over a network. In this hour, then, we'll review logical
topologies.

Logical Topologies

Before discussing topologies again, let's revisit the definition of a
topology. In networking terms, a topology is nothing more than the
arrangement of a network. The topology can refer to the physical layout (which
we discussed in Hour 1, "An Overview of Networking," and really deals
with the wiring, more or less) or the logical layout of the network.

Logical topologies lay out the rules of the road for data transmission. As
you already know, in data networking, only one computer can transmit on one wire
segment at any given time. Life would be wonderful if computers could take turns
transmitting data, but unfortunately, life isn't that simple. Computers
transmitting data have all the patience of a four-year-old waiting in line at an
ice-cream parlor on a hot summer day. As a result, there must be rules if the
network is to avoid becoming completely anarchic.

In contrast to physical topologies, logical topologies are largely abstract.
Physical topologies can be expressed through concrete pieces of equipment, such
as network cards and wiring types; logical networks are essentially rules of the
road that those devices use to do their job. In other words, this is
software.

Ethernet

When packet switching was young, it didn't work very efficiently.
Computers didn't know how to avoid sending data over the wire at the same
time other systems were sending data, making early networking a rather
ineffective technology. Just think about itit was similar to two people
talking on the phone at the same time.

Ethernet, invented in 1973 by Bob Metcalfe (who went on to found 3Com, one of
the most successful networking companies), was a way to circumvent the
limitations of earlier networks. It was based on an IEEE (Institute of
Electronic and Electrical Engineers) standard called 802.3 CSMA/CD, and it
provided for ways to manage the crazy situation that occurred when many
computers tried to transmit on one wire simultaneously.

CSMA/CD Explained

The foundation of ethernet is a method of transmitting data called
CSMA/CD, or Carrier Sense Multiple Access/Collision Detection. It
sounds complicated, but it's quite simple; it's a protocol based on
common sense. Here's how it works, in a blow-by-blow account.

In an ethernet network

All computers share a single network segment, called a collision domain.
A collision domain is the group of computers that communicate on a single
network wire, also called a segment. The segment is a collision domain because
if there's more than one computer in it, it's a cinch that at some
point those computers are going to try to transmit data simultaneously, which is
a big no-no.

Each computer in a collision domain listens to all transmissions on the
wire.

Each computer can transmit data only when no other computer is currently
transmitting.

Each computer listens for a quiet time on the wire (this is the carrier
sense multiple access) in CSMA/CD). When the network wire is quiet (which is
measured in nanosecondsnetwork quiet has no relationship to human quiet),
a computer that has packets of data to transmit sends them out over the network
wire. If no other computers are sending, the packet will be routed on its merry
way.

When two computers transmit packets at the same time, a condition called
a collision occurs (this is the collision detection part of CSMA/CD). In
terms of networking, a collision is the thing that happens when two computers
attempt to transmit data on the same network wire at the same time. This creates
a conflict; both computers sense the collision, stop transmitting, and wait a
random amount of time (in nanoseconds) before retransmitting. The phrase
"random amount of time" is important because it's key to reducing
collisions and it's unlikely that more than one computer on a network will
randomly select the same number of nanoseconds to wait until resending.

The larger the collision domain, the more likely it is that collisions will
occur, which is why ethernet designers try to keep the number of computers in a
segment (and hence a collision domain) as low as possible.

If a second computer tries to transmit data over the wire at the same time as
the first computer, a collisionoccurs. Both then cease
transmitting data, wait a random amount of time for a quiet period, and transmit
again; usually this solves the collision problem. It is really that simple.

CSMA/CD, as personified in ethernet, does have some problems. Sometimes a
network card goes into a mode in which it fails to obey CSMA/CD and transmits
all the timethis is called jabber, and it's caused either by
faulty software or a defective network card. Other problems can be caused by a
segment with too many computers, which causes too many systems to try to
transmit at each quiet time; this can cause broadcaststorms. Fortunately, newer forms of ethernet (specifically
switching, which we'll discuss further on) can circumvent these
limitations by segmenting the network very tightly.

FIGURE
3.1 An ethernet topology: Only one computer can transmit data at a time.

Ethernet's Nuclear Family

Ethernet is broadly used to describe both the logical topology that uses
CSMA/CD and the physical topologies on which CSMA/CD networks run. All the basic
ethernet topologies are described in IEEE standard 802.3. The members of the
nuclear family are listed here:

10BASE-2, or coaxial networking. The maximum segment length of 10BASE-2
is 185 meters. This is a dead technology and is not used for new
installations.

10BASE5, or thicknet. Thicknet is also called AUI, short for
Attachment User Interface. AUI networks are an intermediate step between
10BASE-2 and 10BASE-T. 10BASE5 is a bus interface with slightly more redundancy
than 10BASE-2. The maximum length of a 10BASE5 segment is 500 meters. Like
10BASE-2, this is a dead technology and is not used for new
installations.

10BASE-T, which runs over two of the four pairs of unshielded
twisted-pair wire. In 10BASE-T, the maximum cable length from the hub to a
workstation is 100 meters. 10BASE-T is pretty much dead technology at this
point.

Fortunately, the ethernet standard has grown to include faster networks and
fiber-optic media. The newer members of the ethernet family are described in
IEEE Standard 802.3u, and include these:

100BASE-T, also called fast ethernet, in which data travels at 100
megabits per second over two pairs of unshielded twisted-pair copper wire. The
maximum cable length between the concentrator and the workstation for fast
ethernet is 20 meters, and it requires Category 5 cabling standards. (Cable
standards will be discussed later.)

100BASE-FX and 100-Base-FL, which is fast ethernet running on optical
fibers. Because optical fibers can carry data much further than copper wire,
100BASE-FX and FL have much greater maximum cable lengths than
100BASE-T.

1000BASE-T, also called gigabit ethernet, allows data to travel at one
gigabit (1000 megabits, or ten times faster than 100BASE-T) per second.
Currently, 1000BASE-T is used mostly for servers and for organizational backbone
networks, but over time that will surely change. By the next edition of this
book, it will probably be common to have gigabit ethernet to the desktop. This
topology runs on CAT 5E or CAT 6 copper wire and over fiber.

Token Ring and FDDI

Ethernet CSMA/CD networks provide a relatively simple way of passing data.
However, many industry observers correctly note that CSMA/CD breaks down under
the pressure exerted by many computers on a network segment. These observers are
correct; the squabbling and contention for bandwidth that is part and parcel of
ethernet does not always scale efficiently.

In an attempt to circumvent this problem , IBM and the IEEE created another
networking standard called 802.5. (Does anyone see a pattern here? Every new
invention is built to rectify the older standard's shortcomings.) IEEE
802.5 is more commonly identified with token ring; FDDI also uses the 802.5
method of moving data around networks.

Token ring works very differently from ethernet. In ethernet, any computer
on a given network segment can transmit until it senses a collision with another
computer. In token ring and FDDI networks, by contrast, a single special packet
called a token is generated when the network started up and is passed
around the network. When a computer has data to transmit, it waits until the
token is available. The computer then takes control of the token and transmits
a data packet. When it's done, it releases the token to the network. Then
the next computer grabs the token if it has data to transmit (see Figure
3.2).

FIGURE
3.2 A token ring topology (FDDI works in the same fashion): The only computer
that can transmit is the computer holding the token.

In comparison to the contentious nature of ethernet, token ring and FDDI
appear quite civilized. These two logical topologies do not have collisions in
which multiple stations try to send data; instead, every computer waits its
turn.

Token-Ring Teaching

Token ring's underlying methodology worked to my advantage while a
teaching assistant in grad school. To reduce my tuition, I taught a couple of
sections of freshman composition, and one of the sections was particularly
fractiousstudents would interrupt each other willy-nilly, which got in the
way of teaching and learning. So I brought in a koosh ball (one of those little
rubber hairy plastic things that were all the rage in the early 1990s) and
announced that only the student with the koosh could talk. It took a couple of
classes for the rules to sink in, but once they did, the class was much more
civiland a better place for learning. Who says high technology has no
place in the real world?

Token ring suffers slightly fewer bandwidth-contention issues than ethernet;
it holds up under load fairly well, although it too can be slowed down if too
many computers need to transmit data at the same time.

Despite its load-friendly architecture, IBM held token ring as a proprietary
topology for too many years. By the time it allowed other manufacturers to begin
making devices for token ring, the battle for market share was already lost to
lower-cost ethernet devices. It also never really evolved to transmit data as
fast as ethernet, which might have sealed its fate. Currently, token ring
appears to be dying out one network at a time, and I haven't installed a
new token ring device in about four years.

Asynchronous Transfer Mode (ATM)

ATM networking is the youngest of the topologies described here. It was
designed to circumvent the shortcomings of existing topologies, and hence it was
created from whole cloth. Unlike ethernet, token ring, or FDDI, it can natively
carry both voice and data over network wire or fiber. ATM transmits all packets
as 53-byte cells that have a variety of identifiers on them to determine such
things as Quality of Service.

Quality of Service in packet data is very similar to quality of
service in regular mail. In regular mail, you have a choice of services: first
class, second class, third class, bulk mail, overnight, and so forth. When you
send an overnight message, it receives priority over first-class mail, so it
gets to its destination first.

A few bits of data in a packet of data indicate the quality of service
required for that data. When the Quality of Service feature is
implementedas it is in ATM and Internet Protocol version 6 (IPv6)you
can send packets based on their need for bandwidth. For example, email is
relatively low priority and might be given third-class service; video or audio
content, which has to run constantly, gets a higher priority.

ATM is fast. At its slowest, it runs at 25 megabits per second; at its
fastest, it can run up to 1.5 gigabits per second (which is why phone companies
use it for some of the huge trunk lines that carry data for long distances). In
addition to its speed, ATM is exponentially more complex than either ethernet or
token ring. Most commonly, the 155 megabit per second speed of ATM is used for
applications in which quality of service and extraordinary speed are
required.

Currently, ATM equipment is both esoteric and expensive. Fore Systems and IBM
have both invested heavily in ATM-to-the-desktop technology (that is, they use
ATM to link servers and workstations) and are banking on the need for multimedia
networks over the next several years. ATM standards and interoperability are
still touch-and-go, however.

Unfortunately, ATM, like token ring, has largely been eclipsed in the
consumer market by Fast ethernet and Gigabit ethernet, which provide comparable
performance at lower cost.

That just about wraps up the discussion of logical topologies. Now it's
time to discuss protocols.